Millisecond laser annealing, such as laser spike annealing (LSA), has been widely adopted in advanced semiconductor device fabrication because it offers an ultra-low thermal budget, high dopant activation and super-abrupt junctions. A key challenge to implementing this type of annealing on patterned wafers is the potential large within-chip temperature non-uniformities that can arise from spatial variations in the optical and thermal properties of the wafer surface caused by the features of the IC chip. These adverse effects are referred to in the art as “pattern density effects” or just “pattern effects.”
In one example of LSA, an infrared laser directs a single annealing laser beam to the wafer surface at or near the Brewster angle of incidence and with a P-polarization to minimize reflections and thus any within-chip temperature non-uniformities due to pattern density effects. The infrared wavelength reduces optical interference effects from the patterns because of its relatively long wavelength (e.g., 10.6 microns) compared to the film thickness (e.g., on the order of 1 micron or less). The Brewster angle of incidence is known to be the angle of maximum absorption for a surface and acts to minimize difference in light absorption due to the IC chip features, including the various thin film stacks used in IC chip fabrication.
This single-beam approach works very well for many IC chip features and circuit layouts. However, for certain IC chip features and for layouts involving large features, temperature overshoots have been observed due to optical diffraction at the boundary between two adjacent regions that have different optical properties. This reduces the maximum annealing temperature that can be used to activate the dopants in the adjacent regions.
FIG. 1 is a close-up cross-sectional view of a section of a prior art silicon wafer 10 having a body 9 and a surface 12. The wafer 10 of FIG. 1 includes a feature in the form of an oxide region (e.g., an oxide isolation pad) 16 formed in wafer body 9 adjacent wafer surface 12. The oxide region feature 16 defines an oxide-silicon interface 17 within wafer body 9 and constitutes an example wafer structure or feature. FIG. 2 is a plot of normalized intensity vs. distance x (μm) from interface 17. The plot shows the simulated optical intensity distribution in the section of wafer 10 shown in FIG. 1 during single-beam laser annealing as performed according to the prior art. The simulation was performed using a P-polarized CO2 laser beam LB (see FIG. 1) at a wavelength of 10.6 μm and incident upon wafer surface 12 at an angle of incidence θ near the Brewster angle θB for a silicon substrate (i.e., θ≈θB≈72°). The intensity plot of FIG. 2 shows a relatively strong intensity oscillation in wafer body 9 adjacent interface 17. The periodicity of the oscillation depends on the angle of incidence θ of laser beam LB, and is typically a fraction of the wavelength.
The corresponding temperature distribution in the wafer section is smoother than the intensity distribution due to thermal diffusion, with a typical heat diffusion length for millisecond laser annealing being about 100 μm. However, the temperature at interface 17 is still higher than that of the rest of wafer body 9. This temperature variation is referred to as edge temperature overshoot ΔTedge. This temperature overshoot can lead to edge damage near features formed in wafer 10.